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A. Clarification of the Reaction Pathway

A. Clarification of the Reaction Pathway

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104



Yang and Harris



Figure 18 Transient difference spectra in the CO-stretching region for Re2 (CO)10

(a) in hexane solution; and (b)–(d) in CCl4 solution at various time delays following

295 nm photolysis. In panel (c), bands due to the solvated Re2 (CO)9 CCl4 are

marked by asterisks. Panel (d) is an FTIR difference spectrum before and after

308 nm photolysis. The down-pointing arrows in panels (a)–(c) indicate the CO

stretch of the Re(CO)5 radical. A broad, wavelength-independent background signal

from CaF2 windows has been subtracted. (Adapted from Ref. 72.)



for Re(CO)5 /CH4 . The calculated weak interaction indicates that the mean

thermal energy ¾0.6 kcal/mol at room temperature is sufficient to disrupt

the formation of a stable complex of the form Re(CO)5 (solvent). In other

words, a dynamical equilibrium is established for Re(CO)5 . . . (solvent) $

Re(CO)5 C solvent (10), the time scale of which is on the order of

collision in liquids. This allows the chemically active Re center to undergo

recombination reaction with another Re(CO)5 radical to reform the parent

Re2 (CO)10 molecule.



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Bond Activation Reactions



105



Table 1 Summary of Gas Phase Electron Population Analysis for Re and Cl

Centers Involved in Reaction of Cl Atom Abstraction by Re(CO)5 Radical



Mulliken chargea



Natural chargeb



Re



Cl



Re



Cl



(CO)5 Re



0.147







0.412







Solvated Complex

(CO)5 Re CH4

(CO)5 Re CCl4



0.175

0.189



0.412

0.430





C0.022



Transition State

ts-(CO)5 Re . . . Cl . . . CCl3

ts-(CO)5 Re . . . Cl . . . CHCl2

ts-(CO)5 Re . . . Cl . . . CH2 Cl



0.222

0.244

0.264



0.025

0.0925

0.144



0.481

0.496

0.508



0.168

0.260

0.331



Product

(CO)5 ReCl



0.321



0.209



0.567



0.477



Atomic center



a

b





C0.086



Mulliken charge population for a Cl atom in CCl4 is C0.090 e .

Natural charge population for a Cl atom in CCl4 is C0.068 e .



Geminate-recombination dynamics of the parent molecule provide

further experimental support for the weak Re(CO)5 /solvent interaction.

Shown in Fig. 19 are kinetic traces of the 2071 cm 1 parent bleach in

various solvents. They all exhibit a recovery of about 50% on two time

scales. For example, the parent bleach of Re2 CO 10 in CCl4 recovers on

¾50 ps and ¾500 ps if fitted to a bi-exponential function. The biphasic

recovery can be described by a diffusion model (solid lines in Fig. 19) that

takes into account the dynamics of geminate-pair recombination (71,72).

The physical picture imbedded in the model is illustrated in Fig. 20. In

the figure, the abscissa is the separation of the two Re(CO)5 monomers,

˚ , the contact distance

where the equilibrium Re – Re distance Req ³3.0 A

˚ , and the initial separation r0 ³8.6 A

˚ are marked by short

R ³6.3 A

vertical bars. The portion to the left of R is a schematic potential energy

surface plot depicting dynamics that are not susceptible to the current

model. To the right of R are plots of time-dependent spatial distribution of

a monomer around a recombination center. At short time delays such as 1

and 5 ps, the distribution maxima still linger around the initial separation r0 ,

while the concentration of the recombining monomer builds up very quickly

at R. This accounts for the experimentally observed fast recovery since

the recombination rate kr is defined to be proportional to the concentration



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106



Yang and Harris



Figure 19 Parent bleach kinetics (open circles) of Re2 CO 10 in (a) hexanes,

(b) CCl4 , (c) CHCl3 , and (d) CH2 Cl2 . Fits to a diffusion model to account for geminate recombination are shown as solid lines. Except for the macroscopic viscosity,

identical molecular parameters (see Fig. 20) are used for all the solvents studied.



gradient at the contact distance R. In other words, the fast 50 ps component

can be attributed to a convolution of vibrational relaxation and the probability of reactive collision of the Re(CO)5 pair within the solvent cage to

form the parent Re2 (CO)10 . Driven by the accumulating population around

R, the distribution starts to diffuse away from the recombination center at

time delays longer than ¾20 ps until the system reaches equilibrium. This



Copyright © 2001 by Taylor & Francis Group, LLC



Bond Activation Reactions



107



Figure 20 Illustration for the diffusion model for geminate recombination. Details

are described in the text.



diffusive process is identified with the slower recovery (hundreds of ps) in

the parent bleach data.

In consideration of the above experimental and theoretical evidence,

it is concluded that the reaction, at least the ones that have been investigated, be viewed as proceeding through a weakly solvated 17-electron

Re(CO)5 radical instead of a 19-electron or a charge-transfer intermediate. The fact that no other intermediates are detected prior to the product

formation suggests that the reaction involve only the rate-limiting Cl atom

transfer step.

B. The Nature of the Reaction Barrier — Atom Transfer



This section discusses the correlation of the reaction rate to the chemical

reactivity based on the nature of the transition state. The reaction is carried

out in a series of chlorinated methane solutions, CHn Cl4 n (n D 0, 1, 2).

The free-energy barriers G‡ , determined by the rise time of the product

at 2045 cm 1 , are found to be 5.35 š 0.03 (n D 0), 8.03 š 0.20 (n D 1),

and 8.48 š 0.13 (n D 2) kcal/mol.



Copyright © 2001 by Taylor & Francis Group, LLC



108



Yang and Harris



Figure 21 Transition-state structures computed at the DFT B3LYP level of theory.

˚ Also shown are the imaginary frequenThe Re –Cl and C–Cl bond lengths are in A.

cies associated with each transition state. (Adapted from Ref. 72.)



To better understand the reactivity, the transition states for the series

of reactions were studied using DFT methods. As illustrated in Fig. 21,

the results show that each transition state can be characterized by a single

imaginary frequency that involves simultaneous dissociation of the C–Cl

bond and formation of the Re –Cl bond. The structural variation along the

series of chlorinated methanes suggests that the transition state becomes

more product-like as the number of hydrogen n in CHn Cl4 n increases. For

example, the Re . . . Cl distance in CO 5 Re . . . Cl . . . CHn Cl3 n decreases

˚ n D 0 to 2.84 A

˚ n D 1 to 2.76 A

˚ nD2 ,

monotonically from 2.95 A

˚ for the product (CO)5 Re-Cl. The calculated electron

and finally to 2.52 A

density distribution also displays a similar trend as displayed in Table 1.



Copyright © 2001 by Taylor & Francis Group, LLC



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